The contents of the following Japanese patent application are incorporated herein by reference:
NO. 2015-015457 filed in Japan on Jan. 29, 2015.
1. Technical Field
The present invention relates to a semiconductor device. In particular, the present invention relates to a semiconductor device having a superjunction structure.
2. Related Art
A conventional semiconductor device having a superjunction structure is known in which the impurity concentration changes according to the depth, such as shown in Patent Documents 1 to 4, for example. Furthermore, a semiconductor device having a superjunction structure is known that is provided with a P-type column and an N-type column with different heights, as shown in Patent Document 5, for example.    Patent Document 1: Japanese Patent Application Publication No. 2006-66421    Patent Document 2: International Publication WO 2011/093473    Patent Document 3: Japanese Patent Application Publication No. 2008-91450    Patent Document 4: Japanese Patent Application Publication No. 2004-72068    Patent Document 5: Japanese Patent Application Publication No. 2007-300034
With the conventional superjunction structure, however, the tradeoff between the ON resistance per unit area and the withstand voltage with respect to the PN variation, i.e. the variation between the total amount of n-type impurities in the n-type column and the total amount of p-type impurities in the p-type column, cannot be sufficiently improved.